Graphite and carbon materials, especially reinforced carbon-carbon composite substrate materials, are subject to many applications in modern industry, particularly in the aerospace and aviation fields. However, such materials, when unprotected, are subject to degradation at elevated temperatures. Since many applications involve high temperatures, resistance to high temperature and thermal shock are often required properties of the material.
Reinforced carbon-carbon composite substrates are generally constructed of fibers and bound by a carbon matrix, resulting in a material having excellent structural properties. Generally, carbonaceous fibers such as polyacrylonitrile, rayon or pitch-based fibers are utilized. Carbon-carbon impregnation materials generally are phenolic, furfuryl or pitch-based materials. Densification of the carbonaceous fibers can also be accomplished through use of chemical vapor deposition techniques to deposit carbon and form a matrix strengthens the material. However, the use of a specific substrate material is not a limitation upon the present invention.
Graphite and carbon materials, including reinforced carbon-carbon composites, are subject to degradation, such as oxidation, when utilized in high temperature environments in the presence of oxygen. Generally, an unprotected graphite or carbon material will begin to oxidize at temperatures in excess of about 650.degree. F. in air. Therefore, in order to effectively utilize these materials in high temperature applications, it is necessary to provide protection from degradation, including oxidation. Accordingly, a need exists for a composition of matter and method for forming protective coatings on graphite and carbon materials, and especially for reinforced carbon-carbon composite materials. Copending application Ser. Nos. 251,798, now abandoned and 252,117, now abandoned, describe alumina and boron containing silicon carbide type primary coatings. These coatings are very effective to prevent degradation at temperatures of from about 2500 to about 3000.degree. F. in oxygen containing atmospheres. However, where materials must withstand cycling temperatures which go above and drop below about 2500.degree. F. additional protection against degradation is desirable.
Previously, enhancement coatings, applied to the surface of the primary silicon carbide coatings have been known and described as useful in providing added protection. Enhancement coatings are desirable even where cyclical temperatures will not be a problem. However, it is theorized that under cycling temperature conditions the primary coatings can develop microcracks of fissures which then expose uncoated portions of the carbonaceous substrate to oxidation. Impregnation with tetraethyl orthosilicate (TEOS) which penetrates the microcracks in the primary coating and then, upon curing, converts to silicon dioxide, is known to provide protection to otherwise exposed carbonaceous surfaces. It has also been known to use a silicon carbide paste and liquid alkali silicate materials in combination with TEOS impregnation to provide fairly effective enhancement coatings. However, further improved enhancement coatings are measured, for example, by their ability to reduce material loss under oxidation testing, are highly desirable.